Antares, the brightest star of Scorpius, shines through a narrow gap in the rings of Saturn in this image from the Cassini spacecraft. Antares is a red supergiant, one of the largest stars in the galaxy. It will appear close to the Moon and Venus before dawn on January 29. [NASA/JPL/SSI]

The Moon shoots the gap between two bright companions early tomorrow. They rise about three hours before the Sun, and are in good view at first light. Venus -- the morning star -- is to the left of the Moon, with Antares -- the heart of Scorpius -- about the same distance to the upper right of the Moon.

Although Antares looks smaller and fainter than the other two, that's an illusion -- a result of its distance of about 550 light-years. Antares is actually one of the biggest stars in the galaxy -- big enough to hold 50 million Suns.

Despite its size, Antares is only about 15 to 20 times heavier than the Sun. And a large fraction of that mass is concentrated in the star's core. Its outer layers are so thinly spread that they're little more than a vacuum. In fact, it's hard to tell where the star ends, because it's blowing a strong "wind" of hot gas into space. That wind is almost as dense as the gas at the star's surface.

Antares is so puffed up because of its superhot core. The temperature in the core is billions of degrees hotter than the core of the Sun. At that temperature, the core is radiating so much energy that it pushes strongly on the surrounding layers of gas, "inflating" the star like a giant balloon.

Because the outer layers are so puffy, the surface of Antares is fairly cool, so it looks orange -- a color that's easy to see as you watch Antares and its bright companions before sunrise tomorrow.

Script by Damond Benningfield, Copyright 2010

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